The process of electrophotographic image formation is well known and useful for both analog and digital copying and other reproduction techniques. Generally, the electrophotographic reproduction process is initiated by attaining substantially uniform charging with electricity of a photoreceptive member, and an electrophotographic image forming apparatus is provided with a charging unit for performing the charging.
As a method of the charging, the proximity charging method is cited, in which the charging the surface of photoreceptive member is carried out utilizing proximity discharge induced by the photoreceptive member and a charging member provided opposing thereto.
The proximity discharge is generated in a minute gap in discharge region formed between the photoreceptor surface and the charging member, which are placed either in direct contact with, or in close vicinity to, each other. In the former case of direct contact, some of the region surrounding the contacted portion functions as minute gaps for generating the discharge.
By means of the discharge generated in the minute gap region, the surface of photoreceptive member is charged with electricity.
In the charging method utilizing the proximity discharge, it is known from experimentation that the amount of discharge sufficient for uniformly charging the photoreceptor can be obtained by applying to a charging roller (1) first, solely DC voltage and (2) AC voltage (peak-to-peak) of 2×Vth, in which the value Vth is defined as the discharge onset voltage or the voltage value for initiating a discharge to the photoreceptive member (Japanese Laid-Open Patent Application No. 2001-109238, for example).
It should be noted that the voltage value Vth may be affected by several factors such as the resistance change of the charging roller with environmental conditions, and the change of the charging gap due to dilation of the charging roller also caused by the change in environmental conditions.
This change is illustrated, for example, with the results included in Table 1 obtained from experimentation by the present inventor. In the table, the change in the voltage value Vth is shown as a function of absolute humidity which is taken as a representative of the environmental conditions.
TABLE 1Absolute humidity [g/cm3]0≦ . . . <55≦ . . . <88≦ . . . <1818≦ . . . <2626≦Vth [V]20501840170016701640
Influenced by the change in the voltage value Vth, there may give rise to the case where an AC voltage (peak-to-peak) applied to the charging roller is less than the voltage value of 2×Vth. In such a case, no discharge is generated from the charging roller to the photoreceptor, and the uniform charging of the photoreceptor cannot be attained.
In order to obviate this difficulty it may be contemplated that a higher voltage is applied to the charging roller. However, the higher voltage may make the amount of discharge unnecessarily high, the receptor surface deteriorated and some of its layer scraped off, and discharge reactants and toner additives unduly adhered to the surface, whereby satisfactory image qualities come to be difficult to achieve.
There disclosed in Japanese Laid-Open Patent Application No. 2002-108059 (i.e., the application '059) is environment detection means to be utilized for appropriately changing the AC voltage.
Namely, based on the results obtained by the environment detection means additionally provided, the AC voltage (peak-to-peak) is properly adjusted and applied to the charging roller.
By changing the AC voltage (peak-to-peak) corresponding to the environmental change, therefore, the amount of discharge sufficient for uniformly charging the photoreceptor can be obtained even after the change in the resistance of the charging roller or in the gap between the roller and photoreceptor. In addition, the surface deterioration can be prevented since the amount of discharge is kept not to be unnecessarily high.
In order to perform the environmental detection in the method, however, various AC voltage values corresponding to environmental conditions (temperature and humidity) have to be stored in memory means, this addition of the memory may result in a drawback of costs increase in the image forming apparatus.
On the other hand, it is also known that a photoreceptor can uniformly be charged without influenced by the gap change if the AC current to the photoreceptor is equal to, larger than the value Ivth of the AC current, which flows into the photoreceptor at the onset of the discharge from the charging roller to the photoreceptor.
The application '059 also discloses the method of controlling an AC voltage, in that the AC current Ivth uniformly charging the photoreceptor is obtained experimentally and the AC voltage is applied so as to bring the AC current equal to the value Ivth as standard AC current value.
Specifically, this method is carried out as follows; a predetermined AC voltage is applied to the charging roller during warm-up period prior to image forming operation and an AC current under the applied voltage is measured, and it is subsequently decided whether the thus measured value is the standard AC current value.
If the measured current value is equal to the standard AC current value or less, AC voltage applied to the charging roller is increased and the AC current value under the increased voltage is measured. After examining AC voltages by repeating these steps, the AC voltage applied to the charging roller is set so as to bring AC current value equal to Ivth as standard AC current value.
In the method of the application '059, however, a drawback is realized of taking a relatively long time before setting the AD voltage.
Namely, the steps in the method of determining the AC voltage value for bringing the AC current value equal to the standard AC current value Ivth are performed, as mentioned above, by repeating the following steps such as applying a predetermined AC voltage to the charging roller; measuring an AC current under the applied voltage; deciding whether the thus measured value is equal to, larger than the standard AC current value; increasing AC voltage applied to the charging roller if the measured current value is equal to the standard AC current value or less; and measuring the AC current value under the increased voltage, whereby it takes a relatively long time.
Another method is also known as the means for setting the AC voltage value to be applied to the charging roller such that the AC current flowing into the photoreceptor is equal to, larger than the standard AC current value.
That is, based on a relational expression between the AC voltage and AC current, the AC voltage which yields the standard AC current value Ivth is estimated, and the thus estimated AC voltage value is set as the AC voltage value to be applied to the charging roller.
It should be noted that the relation between the AC voltage and AC current may be affected by several factors such as the resistance change of the charging roller and the change in the charging gap.
As a result, even after applying the estimated AC voltage to the charging roller, there gives rise to the case where the AC current cannot be brought to be equal to the standard AC current value Ivth, as anticipated.
This gives rise to several difficulties such as the generation of unduly high amount of discharge, the concomitant deterioration of the surface of photoreceptor, or failure in uniform charging of the surface as a result of unsuccessful discharge.